The statement that is INCORRECT about Ellingham diagram is

To decide which statement about the Ellingham diagram is incorrect, we first recall what an Ellingham diagram actually represents. The Ellingham diagram is a plot of the standard Gibbs free-energy change $$\Delta G^{\circ}$$ for various oxidation (or reduction) reactions versus temperature $$T$$. Its abscissa (horizontal axis) is temperature and its ordinate (vertical axis) is $$\Delta G^{\circ}$$, usually expressed in $$\text{kJ mol}^{-1}$$.

Because of its construction, the Ellingham diagram directly relates to the thermodynamic feasibility of reduction or oxidation processes. We have the fundamental thermodynamic relation

$$\Delta G^{\circ} = \Delta H^{\circ} - T\Delta S^{\circ},$$

which tells us whether a given reaction is spontaneous (when $$\Delta G^{\circ} < 0$$), at equilibrium (when $$\Delta G^{\circ} = 0$$) or non-spontaneous (when $$\Delta G^{\circ} > 0$$). The Ellingham lines show how $$\Delta G^{\circ}$$ changes with temperature, so they immediately give information about the temperature above which a metal oxide can be reduced by another reducing agent whose $$\Delta G^{\circ}$$ line lies below.

Thus, from the diagram we can clearly obtain:

1. Information about the free-energy change of reactions (because each line is literally a plot of $$\Delta G^{\circ}$$).
2. Information about the reduction of metal oxides (because comparison of two lines tells whether the reducing agent can reduce the oxide).
3. Some insight into phase changes during the reaction if the slope or position of a line changes abruptly; such kinks generally correspond to a change in the phase of reactants or products, for example from solid to liquid.

However, one must remember that thermodynamics and kinetics are separate subjects. Thermodynamics tells us whether a reaction is possible, whereas kinetics tells us how fast it proceeds. The Ellingham diagram is purely thermodynamic; it says nothing about activation energy, mechanism, or the rate at which the reduction or oxidation actually occurs.

Therefore, the statement saying the Ellingham diagram “provides idea about the reaction rate” is incorrect because reaction rate is a kinetic concept, not described by $$\Delta G^{\circ}$$ versus $$T$$ plots.

So, among the given options:

Option A (provides idea about the reaction rate) is the incorrect statement.

Hence, the correct answer is Option A.